1. Field of the Invention
The present invention relates to an uninterruptible power supply apparatus for constant commercial power supply, which supplies power via an AC switch while a commercial power source is normal, and turns off the AC switch, in the event of a power failure in the commercial power source, operates a power conversion device to convert DC power, which is stored beforehand in a storage battery, to AC power, and supplies the AC power to a load.
2. Description of Related Art
A conventionally known uninterruptible power supply apparatus is disclosed, for instance, by Japanese Patent Laid-Open No. 260684/1993 (Patent Document 1). Patent Document 1 describes a CATV (cable television) uninterruptible power supply apparatus, which comprises a direct power supply circuit for receiving commercial AC power from its source and outputting AC power for constant power supply to a load; a commercial power input abnormality detection circuit for detecting an abnormality in the received commercial AC power; and an inverter circuit for outputting AC power for power supply from a battery to a load immediately upon receipt of a commercial power input abnormality detection signal, which is generated by the commercial power input abnormality detection circuit when it detects an abnormality in the commercial AC power. The commercial power input abnormality detection circuit comprises a first detector, which outputs a commercial power input abnormality detection signal when the voltage of the received commercial AC power is lower than specified, and a second detector, which outputs a commercial power input abnormality detection signal when the voltage of the received commercial AC power is higher than specified.
If the uninterruptible power supply apparatus described in Patent Document 1, which supplies power mainly from a commercial power source, detects abnormalities by a conventional method for input voltage detection, a problem indicated in FIG. 4 arises. In FIG. 4, the vertical axis represents detection voltage settings and actual output voltages from an AC switch, whereas the horizontal axis represents a load factor. When the load factor is low, the requirements specifications for the load are complied with because an insignificant voltage drop is invoked by an impedance exhibited by the wiring to the power-receiving end of the apparatus and an AC switch within the apparatus. However, if the load factor is high, there are some cases where the requirements specifications for the load are not complied with.
Conventionally, AC switch restoration control has been exercised in accordance with the root mean square value of an AC voltage that is input from a commercial power source to the AC switch. Therefore, when the load is close to 100% and the hysteresis width is not adequate for lower-limit detection as indicated in FIG. 4 because the output voltage is decreased by a specified amount of voltage from the input voltage due to the impedance of the input wiring for the AC switch, detection chattering occurs.
Immediately before restoration, the uninterruptible power supply apparatus supplies power from a battery to the load via an inverter. It means that the load is not connected to the input voltage (commercial power source). When restoration is achieved by a conventional detection method that uses the input voltage only, the prevalent load factor is not known. Therefore, it is impossible to determine the degree of a voltage drop that is to be caused by the above-mentioned impedance. Consequently, the restoration voltage detection value has to be set in terms of a maximum load factor. As a result, if the restoration voltage is set in compliance with the load specifications, the input voltage variation range narrows, giving rise to a problem.